Gold cyanidation in Gold Mining

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Gold cyanidation, also known as the cyanide process or MacArthur–Forrest process, is a hydrometallurgical method for extracting gold from low-grade ore by converting it into a water-soluble coordination complex. Widely used in gold extraction, cyanidation is also employed in silver extraction after froth flotation. Over 70% of global cyanide consumption is attributed to the production of reagents for gold recovery, making gold the primary driver of this technology. Despite its efficacy, the process is controversial due to the highly toxic nature of cyanide, leading to bans in some regions. However, when applied with proper safety measures, cyanide remains crucial in the gold mining industry. Adequate pH control, usually achieved with lime, is essential for safe cyanide use.

The history of gold cyanidation dates back to 1783 when Carl Wilhelm Scheele discovered gold’s solubility in cyanide solutions. Researchers like Bagration, Elsner, and Faraday determined the stoichiometry of the soluble compound, revealing that each gold atom required two cyanide ions. John Stewart MacArthur, in collaboration with Robert and William Forrest, developed the MacArthur–Forrest process in 1887 to address challenges in extracting gold from pyritic ore. By suspending crushed ore in a cyanide solution, this process achieved up to 96% pure gold separation. Its application on the Rand in 1890 sparked a gold mining boom, despite operational imperfections. Gilbert S. Peyton further refined the process in 1891 at the Mercur Mine in Utah. Later improvements, such as the Merrill–Crowe process by Charles Washington Merrill and Thomas Bennett Crowe around 1900, enhanced the treatment of cyanide leachate using vacuum and zinc dust.

Chemical Reactions in Gold Cyanidation:

The gold cyanidation process involves several chemical reactions, the primary one being described by the “Elsner equation”:

[ 4 \text{ Au} + 8 \text{ NaCN} + \text{O}_2 + 2 \text{ H}_2\text{O} \rightarrow 4 \text{ Na[Au(CN)}_2\text{]} + 4 \text{ NaOH} ]

This equation represents the dissolution of gold ((\text{Au})) in the presence of sodium cyanide ((\text{NaCN})), oxygen ((\text{O}_2)), and water ((\text{H}_2\text{O})), forming the soluble gold complex (\text{Na[Au(CN)}_2\text{]}) along with sodium hydroxide ((\text{NaOH})). Potassium cyanide and calcium cyanide can substitute for sodium cyanide in this process.

The soluble gold species formed is dicyanoaurate, which can be recovered by adsorption onto activated carbon.

Application:

  1. Ore Processing: The ore is comminuted using grinding machinery and may undergo further concentration by froth flotation or centrifugal (gravity) concentration.
  2. Cyanidation: Sodium cyanide or potassium cyanide, and sometimes more cost-effective calcium cyanide, are added to the ore slurry. To prevent toxic hydrogen cyanide formation, slaked lime (calcium hydroxide) or soda (sodium hydroxide) is included to maintain a strongly basic pH (above 10.5).
  3. Lead Nitrate: Lead nitrate can enhance gold leaching speed and recovery, especially in treating partially oxidized ores.
  4. Dissolved Oxygen: Oxygen is essential for cyanidation, and a deficiency slows leaching. Air or pure oxygen can be purged through the pulp to maximize dissolved oxygen. Pre-aeration of ore in water at high pH can improve cyanidation efficiency, especially for partially sulfidized ores.

Recovery of Gold:

  1. Carbon in Pulp: Adsorption of gold onto activated carbon.
  2. Electrowinning: Electrodeposition of gold from solution.
  3. Merrill–Crowe Process: Precipitation of gold using zinc dust.
  4. Cyanide Remediation: Processes to detoxify cyanide-containing waste streams, including INCO-licensed and Caro’s acid processes that convert cyanide to less toxic forms like cyanate.

Environmental Considerations:

Cyanide remaining in tailings from gold plants is potentially hazardous. Detoxification steps are taken to lower cyanide concentrations, often through processes like INCO-licensed or Caro’s acid, converting cyanide to less toxic forms like cyanate, which can further hydrolyze to ammonium and carbonate ions. Remaining free cyanide can be reduced to meet regulatory standards. Studies emphasize the persistent release of toxic metals into groundwater and surface water from residual cyanide in gold-mine tailings.

Effects on the Environment:

Gold cyanidation, despite being widely used (90% of gold production), is controversial due to the toxic nature of cyanide. While aqueous cyanide solutions degrade rapidly in sunlight, less-toxic products like cyanates and thiocyanates may persist for years. Famous cyanide spills, such as those from Summitville, Ok Tedi, Omai, Baia Mare, and others, have had devastating effects on rivers, causing widespread ecological damage downstream. Although affected areas can be repopulated over time, the impact on aquatic life can be severe.

Notable cyanide spills have led to protests against new mines, like Roşia Montană in Romania, Lake Cowal in Australia, Pascua Lama in Chile, and Bukit Koman in Malaysia.

Alternatives to Cyanide:

Due to cyanide’s toxicity, there is ongoing research into alternative methods for gold extraction. Some alternatives include thiosulfate, thiourea, iodine/iodide, ammonia, liquid mercury, and alpha-cyclodextrin. Challenges include reagent cost and the efficiency of gold recovery. Thiosulfate has been commercially implemented for ores containing stibnite, and glycine-based lixiviants are also being explored.

Legislation:

Several regions and countries have banned cyanide mining due to environmental concerns. US states like Montana and Wisconsin, along with the Czech Republic and Hungary, have implemented such bans. The European Commission rejected a proposal for a ban, stating existing regulations provide sufficient environmental and health protection. In Romania, attempts to ban gold cyanidation were rejected by the Parliament, leading to protests calling for a ban.

In the EU, the Seveso II Directive and the Groundwater Directive (replaced by the Water Framework Directive) control the industrial use of hazardous chemicals, including cyanide. In response to the 2000 Baia Mare cyanide spill, Directive 2006/21/EC on the management of waste from extractive industries was adopted. It sets limits on weak acid dissociable cyanide concentrations and requires financial guarantees for mine cleanup. The industry has introduced a voluntary “Cyanide Code” with third-party audits to reduce environmental impacts.

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